The present invention relates to a system and a method for starting up stirring machines in a sediment in a controlled manner.
If a stirring means sits in a solid matter sediment, “starting up in a sediment” is what is spoken of. This case may occur if a stirring machine has stopped, either due to operative causes or due to a disturbance, e.g. a power breakdown, contrary to requirements, while processing suspensions. The solid matter sinks downward in the container and forms a sediment. Depending on solid matter content of the suspension and on a position of stirring elements, the sediment covers the stirring element or elements. In dependence of a period length of a standstill, the sediment subsequently densifies. Starting up following standstill thus becomes more difficult up to impossible.
For starting up stirring machines in a sediment the following proposals have been made so far:
It has been tried to position stirring elements above a sediment or to reduce the sediment level, such that the stirring elements are not stuck in the sediment. However, for achieving satisfactory stirring, stirring elements generally need to be positioned in the bottom third of a container. Deviating too far from this positioning is usually not admissible as this does not make any sense with the actual task of stirring, namely “suspending”. A reduction of the sediment level would require of an operator of an installation to process less material or to process a suspension having less solid matter content, which would not provide any commercial benefit.
It may also be envisaged that, for supporting a startup of stirring machines in a sediment, purging by air is carried out, which could be done, for example, by way of gassing lances if present. In many cases, pressurized air that is at hand could be used anyway. In the case of an application with a gassing device below the stirring element, this device can also be used. There is, however, the disadvantage herein that air is compressible and will bubble upwards in an unspecific fashion. Furthermore, the openings of the gassing system are susceptible to clogging. Purging by air moreover fails if lances are arranged above the stirring element.
There is also the simple option to empty the container, such that starting up stirring machines in a sediment is not required at all. This method, even if fail-safe, has the disadvantages that it takes much time, and that liquid having a high degree of acidity or alkalinity, as well as a sediment, have to be transferred away and stored in an interim place.
Another possibility is given in making the stirring element rotate backwards, which reverse operation is easily achieved providing a frequency inverter or a pole reversal. There is, however, the disadvantage that a reduction of the starting-up torque depends on the type of stirring element and, in particular, on the blade geometry. Blades could break due to their shape and mechanics not necessarily being designed for the reverse direction.
Furthermore, emergency power units could be provided for stirring machines. One or several stirring machines can be operated simultaneously or alternatingly, such that solid matters are not deposited. However, for many major stirring machines providing emergency power units is connected with huge investment costs. Intermittingly operating a plurality of stirring machines yields the advantage that the emergency power supply may be designed smaller, but has the disadvantage that re-switching of a stirring machine only works if the depositing periods have a sufficient length. For example, even in big containers having a filling level of 10 to 20 m and with a high particle sinking speed of 1 to 10 cm per sec, the sediment is formed in a few minutes.
U.S. Pat. No. 7,331,704 B2 describes a draft tube arrangement with a stirring element. The stirring element is arranged in such a way that it is situated above the sediment in the case of a standstill. The draft tube is slit in its lower region to allow easy purging.
DE 34 42 294 A1 describes a device for homogenization and suspension of solid matter turbidities in a basin. In the device a pneumatic and/or hydraulic starting-up aid is provided which facilitates starting up the stirring blades in the case of sedimented solid matters. The starting-up aid consists of pumps and nozzles. It is described how, by means of the pumps and the nozzles, by suctioning-in liquid from the upper region of a turbidity and pressing this liquid, by corresponding pipelines and pumps through the nozzles, into the lower region of the basin respectively of the container, the sediment can be disaggregated and the stirring blades are actuated only after sufficient disaggregation of the sediment.